In mobile radio communications systems, the signal propagation between a transmitter and a receiver can be understood by introducing the concept of a mobile radio channel impulse response that introduces a filtering action on the signal. For example, the baseband impulse response, h(t), can be expressed as:                               h          ⁢                      (            t            )                          =                              ∑            i            N                    ⁢                                    χ              i                        ⁢                          ⅇ                              jϕ                i                                      ⁢                          δ              ⁢                              (                                  t                  -                                      τ                    i                                                  )                                                                        (        1        )            where χi is the amplitude of the ith received signal, φi is the phase shift of the ith received signal, δ is the impulse function, and τi is the time delay of the ith received signal. Equation (1) shows that the received signal can be thought of as a series of time-delayed, phase-shifted, and attenuated versions of the transmitted signal. If the channel is time variant, then χi, φi, and τi are also functions of time.
Radio propagation is characterized by multiple reflections, diffractions, and attenuations of the signal caused by natural obstacles like buildings, hills, etc. which result in multi-path propagation. Accordingly, radio waves arrive at a mobile receiver from different directions and with different time delays, and are made up of a possible direct ray and reflected rays. Scattered reflections cause Rayleigh fading and have very small mutual delay differences that are not resolvable in the receiver. These rays (which have a magnitude and a phase) combine vectorially at the receiver to give the resultant signal, which depends on the phases and amplitudes of the reflections that exist in the multi-path field.
As a mobile radio moves from one location to another, a Doppler effect occurs. In general, the Doppler effect relates to a change in the apparent frequency of a source of electromagnetic radiation when there is a relative motion between the source and the observer. For moving mobile radios, where multi-path propagation is common, the phase relationship between the components of the various incoming rays changes, so the resultant signal is spectrally spread. This phenomenon is referred to hereafter as Doppler spreading. Whenever relative motion exists in the absence of multipath propagation, there is a Doppler shift of the frequency components of the received signal, but no spreading of the received signal spectrum. This phenomenon is referred to as Doppler shifting. This present invention is directed to estimating Doppler spread.
A multi-path signal envelope is characterized by a distribution function of amplitude that follows the so-called Rayleigh distribution function, which is why multipath is sometimes referred to as Rayleigh fading. When a mobile radio moves in a multipath environment, the received signal appears to vanish, i.e., “fade,” at certain positions. However, moving a few meters brings it back again. Thus, a mobile radio moving in a multipath environment experiences signal fluctuations, and this effect is referred to as Rayleigh or fast fading.
FIG. 1 illustrates a schematic diagram of a communications system 10 with a transmitter 12, transmitting information over a carrier frequency fc on a radio channel that is subject to fading a(t) and noise n(t). The received signal is received on a frequency that is offset from the transmit carrier frequency. The time-varying, complex fading coefficient a(t) models fast fading. In essence, the Doppler spread describes how fast the channel is changing, or equivalently the spreading of the received signal caused by Rayleigh fading. More formally,fd=fc·ν/c  (2)where fd is the Doppler spread, fc is the carrier frequency, ν is the mobile velocity, and c is the speed of light.
Third generation wireless cellular communications systems, such as the Universal Mobile Communications Systems (UMTS), must support communications with a mobile station traveling at a considerable velocity, e.g., up to 500 km/h, over Rayleigh fading radio channels. The resulting Doppler spreading on such a channel leads to performance degradations absent suitable compensation. To be able to track the channel accurately, knowledge of the Doppler spread is needed. One variable that affects the accuracy of such an estimation is a frequency offset caused by differences between local oscillators in the transmitter and receiver.
FIG. 2 illustrates Doppler spreading and frequency offset errors. More particularly, FIG. 2(a) shows a transmitted signal corresponding to a narrow frequency spectrum. FIG. 2(b) shows that transmitted signal spectrum much wider after Doppler spreading by a fast fading channel. In addition to the Doppler spreading in the frequency domain, where fd shows the maximum Doppler spread of the spectrum, there is a frequency error or offset that shifts the whole frequency spectrum. FIG. 3 illustrates phase changes of the received signal caused by Doppler spreading over a fast fading channel with a frequency offset. The slope (dashed line) of the phase is greater than zero represents a positive frequency offset. The unpredictable variations in the phase are caused by Rayleigh fading. When a Rayleigh fading signal is received, it can be viewed as a sum of a number of incoming waves, each with its own amplitude, frequency, and phase. The vector sum of these waves looks in the frequency domain like FIG. 2(b), and in the time domain like FIG. 3. In order to accurately determine the Doppler spreading of a fast fading channel, any frequency offset should be compensated before estimating the Doppler spreading.
Accurate Doppler spread estimation can be quite useful in mobile communications systems. First, knowledge of the Doppler spread can be used to improve the performance of the demodulator and reduce bit error rates. Second, accurate Doppler spread estimations may be used to optimize interleaving links in order to reduce reception delays. Third, in a cellular system with hierarchical cell layers, the Doppler spread may also be used by the network in cell layer assignment strategies. For example, low speed mobile radios would be assigned to pico cells, medium speed mobile stations to micro cells, and high speed mobile stations to macro cells. By assigning high speed mobiles to large cells, the number of handovers can be reduced. This reduces the amount of signaling, and therefore improves system capacity.
A fourth example use for accurate Doppler estimation is to adjust the filtering bandwidth for channel estimation in the receiver, which is one way to improve demodulator performance. When a narrowband signal is transmitted over a Rayleigh fading channel, its spectrum spreads. After this spectral spreading, it looks like FIG. 3 in the time domain. In order to coherently detect the data symbols, a coherent receiver has to track these unpredictable changes in the phase. This tracking (often called channel estimation) is usually performed using some kind of filtering, with the goal to filter out as much noise as possible. With knowledge of the Doppler spread, the filtering can be adjusted to pass the useful signal and filter out the noise outside the Doppler spectrum.
A fifth example use would be to set an appropriate power control step in a wideband CDMA-type mobile communications system. For a Rayleigh fading channel with a large Doppler spread, the channel power changes rapidly. Power control tries to compensate for these channel changes. Therefore, for a rapidly changing channel (i.e., high Doppler spread), it can be advantageous to use a larger power control step size than for a slowly fading channel.
These and other benefits are achieved by estimating a Doppler spread associated with a Rayleigh or fast fading channel established between a transmitter and receiver, e.g., a base station and a mobile station. In particular, the Doppler spread is estimated through calculation of the autocorrelation function of a sequence of complex channel estimates determined from the known sequence in a received signal. More specifically, a sequence of complex channel estimates obtained from the known sequence in a first sampling interval is complex conjugated and then correlated with a sequence of complex channel estimates obtained from the known sequence in a second sampling interval which have been not been complex conjugated. A zero crossing of the complex autocorrelation function is detected, and the estimated Doppler spread is calculated using this zero crossing and a Bessel function.
In a preferred, non-limiting embodiment, the known sequence is compensated for a frequency offset. The estimated Doppler spread and the compensated known sequence may be used to estimate the Rayleigh or fast fading channel. The estimated channel response is used to filter an unknown sequence in the received signal to compensate for phase error caused by the Doppler spreading over the fast fading channel.